Artificial hip joint prostheses are widely used today, restoring mobility to patients affected by a variety of conditions, particularly arthritis. The satisfactory performance of these devices can be affected not only by the design of the component itself but also by the final placement and geometry of the implanted component and by the long-term fixation of the device. Improper placement or positioning of the device or an improper fit to the patient's anatomy can adversely affect the goal of satisfactorily restoring the clinical biomechanics and function of the joint.
The primary role of the artificial hip prosthesis is to restore the diseased and/or damaged joint to normal function. This function causes significant forces such as axial, bending, and rotational forces, to be imparted to the device. The component must remain adequately fixed within the medullary canal while it endures these forces, because adequate fixation of the component is necessary to ensure proper functioning and a long useful life of the artificial hip component. Early designs of artificial hip components relied primarily on cemented fixation. These cements, such as polymethylmethacrylate, were used to anchor the component within the medullary canal by acting as a grouting agent between the component and the endosteal (inner) surface of the bone. While this method of fixation by cement provides immediate fixation and resistance to the forces encountered and allows the surgeon to position the device effectively before the cement sets, it is not without problems. Over time the mechanical properties and the adhesive properties of the bone cement degrade; eventually the forces overcome the cement and cause the components to become loose due to a failure at the cement/bone or cement/stem interface.
Alternative approaches to address the issue of cement failure include both biological ingrowth and press-fit stems, separately and in combination. Stems designed for biological ingrowth typically rely on the bone itself to grow into a specially prepared surface of the component. The approach firmly anchors the device within the medullary canal, but it does not result in immediate fixation of the component because it takes time for the bone to grow into the specially prepared surface. Press-fit stems may or may not have specially prepared surfaces and typically rely on some degree of interference fit of the component within the bone's medullary canal to achieve stable fixation. One particular type of press-fit stem is tapered in one or more planes such that the degree of press-fit of the stem into the medullary canal increases as the stem is more deeply seated into canal. While a tapered stem design has the advantage of reliably producing a stable press-fit condition in the bone, provided the stem is properly sized for the particular bone, the final position of the stem will depend on a number of variables, including bone geometry, bone quality, stem geometry, and surgical technique.
The hip head center of rotation is determined by the head position because typical hip heads are spherical. In most devices the head position is determined by the stem position because the two are connected through an integral neck. Many devices in existence use modular hip heads to increase or decrease neck length, which alters both head height and head offset proportionately and simultaneously. The neck portion of the device that is attached to the stem receives the modular heads. This results in the head position being integrally linked and thus aligned with and determined by, the stem portion. Multiple positions of the heads are accomplished by using hip heads with various bore dimensions and extended or reduced offsets or skirts which limit the positioning of the head to the angled neck axis. In many instances this may not be appropriate as one may only wish to increase offset while maintaining head height (or vice versa), which can not be accomplished with the modular head type devices previously described. In addition, one could not address anteversion of the neck in such a device as described. The amount of anteversion is determined by the angular difference between the stem-axis/neck-axis plane to that of the coronal plane. Since the head position is directly linked to the stem position, anteversion can only be achieved by sacrificing stem position by rotating the stem.
Some devices incorporate modular components, such as modular stems with modular sleeves, or modular proximal and distal portions of the stem, to provide some degree of adjustability for the final stem geometry. This adjustability may or may not include lateral offset, leg length, and/or anteversion, depending on the specifics of the design and on the available component. Such devices have two basic means of connection, tapers and threads, used alone or in combination. Taper connections have the disadvantage that the final axial position of the two components, relative to each other, is dependent on the precise geometry of the tapers; deviations in geometry within the tolerances allowed for manufacturing results in deviations in the final axial position of the modular component with the tapered connection. The strength of the coupling between the components with the tapered connection is also in part dependent on the level of force used to assemble the components. Similarly, threaded connections have the disadvantage that the strength of connection is in part dependent on the magnitude of torsion applied to the threaded coupling mechanism during assembly. Insufficient impaction force for tapered connections, and/or insufficient torsion for threaded connections, applied during assembly can leave the assembled component at risk of disassembly during the functional lifetime of the device. Unintended disassembly of implanted components is a serious complication that generally requires medical intervention ranging in severity from closed manipulation to surgical revision. This can be a significant risk for tapered and/or threaded coupling means especially considering that the assembly is accomplished in the operating room, rather than under more controlled conditions such as in a factory, in order to take full advantage of the modularity. Thus a design that provides a coupling means for the modular components that has a more reproducible final geometry and reproducible strength of connection, that is less dependent on the surgeon, operating room staff, or other persons acting outside the place of manufacture, would be of significant benefit.